The body's immune system makes antibodies against foreign substances (such as bacteria and viruses); however, in some diseases (called autoimmune diseases), the body's immune system makes antibodies against itself. SLE is an autoimmune disease. In SLE, antibodies are often made against kidney cells, damaging the kidneys and causing serious health problems. Some antibodies (but not all) are made when certain cells in the immune system connect to each other. A protein called CD40 ligand (CD40L) is often needed to connect cells together (CD40L is found on the outside surface of some cells). IDEC-131, an experimental product that is a humanized form of a mouse antibody, attaches to the CD40L on a cell. In doing so, IDEC-131 prevents the cell from connecting to other cells, and this prevents some types of antibodies from being made. It is thought that the interruption of this pathway may have effects on the immune system that will result in improvement in the signs and symptoms of SLE. The purpose of this project is to determine the safety, tolerability, and effectiveness of an investigational antibody, IDEC-131, in the treatment of SLE.